How to match preamp gain to input sensitivity

Discussion in 'Amps [BG]' started by Milestones, Jun 29, 2013.


  1. Milestones

    Milestones

    Joined:
    May 28, 2012
    I know this has been covered before, but I Google'd and FAQ'd and I'm still confused.

    So I know the preamp output has to be similar to the power amp's input sensitivity to drive the power amp fully. My problem is that in spec sheets for preamps, I've only seen their gain listed in db. Input sensitivity is measure in Vrms. Everything that talks about preamp output matching input sensitivity lists the preamp output in Vrms. Where is that on all the spec sheets? Or can you calculate it based on the db gain?
  2. Lonesomedave

    Lonesomedave

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    are you talking about equipment that you own or are thinking of buying?

    most pre amps are designed to work with a wide range of available amps

    my son's pre amp is a UA 610, and we have never had any problem working with any of several available bass amps...he currently uses both an acoustic B450 and a fender super bassman

    if you are planning a rig and you want to be sure, call the manufacturers and tell them what you plan to do, and they will tell you whether your proposed rig is a compatible match up.

    if you want to run from the pre amp to a power amp (say, a crown or a carvin), i don't know, but i have a feeling you will be fine whatever your choices

    [​IMG]
  3. Milestones

    Milestones

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    May 28, 2012
    I'm just trying to understand everything better. I'm talking about a preamp running into a power amp in a rig though. Your son wouldn't have to worry about matching the input sensitivity because he's using heads that contain both the preamp and power amp sections.
  4. Lonesomedave

    Lonesomedave

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    understood...though he does use the UA as a pre for both....

    however, if you are running a pre amp into a power amp, that is what they are made for...my advice is still the same...call the makers and ask, but i do not think you're going to run into any problems

    [​IMG]

    ps-edit....in fact, before we bought, that was one option that Ray, of Corner Music here in Nashville recommentded....use the UA 610 as a pre to a crown power amp...he never discussed the sensitivities of either....and believe me....Corner Music and Ray are well respected in the Nashville music community for giving good advice
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  6. bongomania

    bongomania Gold Supporting Member

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    The truth is that preamp and power amp manufacturers have a wall of silence between them. Ask a preamp maker and they'll say their preamp should work with any power amp, and a power amp maker will say the same thing about preamps. But they are wrong--they just won't admit it, and cannot acknowledge the possibility. Even Bob Lee, who I respect highly, has an unexplainable blind spot in this area.

    Check out the faq articles about it linked in my sig. No easy answers are there, but you'll find as much info as there is to help weed through the utter lack of support provided by the gear makers.
  7. Jeff Scott

    Jeff Scott Don't just TalkBASS - PlayBASS! Supporting Member

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    Virtually all preamps have more than enough gain to drive most power amps to full output. Now, if you can be more specific as to the particular preamp and/or power amps you are considering/already have we can be far more helpful.

    Garbage in/garb........;)
  8. Downunderwonder

    Downunderwonder

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    There are standard voltages referenced when dBv and dBu are stated. Google convert dBu dBv to get the goods.
  9. Downunderwonder

    Downunderwonder

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    To complicate matters a balanced connection uses dual signals on opposite phase summed to cancel noise. Using an unbalanced output into a balanced input needs double the voltage for an equivalent strength signal.
  10. bongomania

    bongomania Gold Supporting Member

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    Not quite true. You can "make it true" on paper by playing around with the numbers, and you can force the issue by cranking all EQ's up and running the input gain into distortion. But if you start only with a typical medium-output bass guitar input, use only minimal EQ, and avoid clipping, you'll find that very many bass-specific preamps cannot reach the 1.4v RMS that many power amps need.

    Weirdly, this is never a problem for pro audio mic preamps and channel strips, which are designed for the +4dBu standard. Most bass preamps, however, are designed to a -10dBu standard, set in ye olde days by Fender and Ampeg.
  11. Vince Klortho

    Vince Klortho

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    Not true. The sum of two signals in opposite phase is zero.


    From the wikipedia :

  12. Vince Klortho

    Vince Klortho

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    It can be calculated. I saw this in a post not too long and saved it because it is very useful. I don't have a link to the post but it can be found easily enough if you are curious. Anyway, here it is :

    FWIW, 3.46V PTP translates to 1.22V RMS (divide by 2.828 or 2 X sqrt(2)).
  13. Downunderwonder

    Downunderwonder

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    Afaik bass preamps with balanced outs send symmetrical opposing phase signals so the common or garden variety power amp or remote mixer can flip the phase on one leg thereby summing any interference to zero. More than I wanted to type on my phone earlier.
  14. MAMMOTHvolume

    MAMMOTHvolume

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    The input sensitivity is the input voltage required to reach maximum output. I'd worry more about the input and output impedance's!
  15. bongomania

    bongomania Gold Supporting Member

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    The problem is that the dB spec given for most rack preamps is their total gain in dB, not their RMS output in dBu or dBv or other reference-based measure. The reason for this is that the RMS output is wholly dependent on the input signal, whereas the total gain is about the same at any input level (before hard clipping). So it's just easier to spec gain without having to explain about the meaning of RMS, peak, reference voltages, etc.
  16. Milestones

    Milestones

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    Bongomania, that's exactly what I was confused about. But it seems like a lot of people here know a preamp's Vrms output but I can never find the spec. For example, take the Ampeg SVP-CL. The general consensus here is that the output is too low to drive most power amps fully unless the power amp has a low input sensitivity. On many other pro audio forums, they also seem to know a preamp's output in Vrms. How do they know that? Do they measure it themselves with a multimeter or something?

    If I understand correctly, a spec given in dBv or dBu is a measurement in regards to a reference point and can be converted to Vrms, but a spec given in plain dB cannot be converted. Is that right?

    If the Vrms output is totally dependent on the input signal, then is there a way to generalize and say that for a passive bass, a preamp should have X amount of dB gain, and for active basses depending on the onboard preamp spec, it should have Y amount of dB gain?
  17. RickenBoogie

    RickenBoogie

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    Well, it's still not quite that simple, because even passive basses are not all the same. Some in fact, are hotter than some active basses. We need standards.
  18. bongomania

    bongomania Gold Supporting Member

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    Amen!
  19. Milestones

    Milestones

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    So is testing with a multimeter and your own bass the only way to get that spec?

    Maybe a better question is why are we at all concerned about the preamp's output level or gain if it's ultimately the input signal that determines it?
  20. Foz

    Foz

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    The actual RMS output in dBu or dBv isn't a spec its a result that will very with every note you play. X level of input with Y settings on the unit will yield Z level of output. So - yeah - if you want to know wants happening you have to have some sort of metering. Some preamps do an excellent job of providing metering at multiple places along the path through the unit...

    [​IMG]

    others make you use your ears [which works pretty good as long as the unit has a reasonable range and critical listening is inside your comfort zone].

    Also a number of modern power amps do a very good job of metering input levels = some of the DSP one's will even do peak/hold metering so you can see how close you are to hitting the stops. :hyper:

    ===

    The relevant actual "specification" is the range of input levels the unit can handle without clipping and the resulting range of clean output in dBu/dBv.

    As BM points out this aint rocket surgery = channel strip / mic preamp manufacturers manage to provide this information without fanfare.

    Example:

    http://www.rane.com/ms1s.html#gpm1_5

    Some instrument preamp manufacturers provide similar information... some don't = IMO a solid clue you shouldn't buy one without advice from someone who has measured/metered the thing and kinda half-ass knows what they are talking about.
  21. Jeff Scott

    Jeff Scott Don't just TalkBASS - PlayBASS! Supporting Member

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    Yeah, I should not have said most preamps, as a lot of MI stand-alone preamps do not adhere to the same standard that most studio type preamps follow. Putting something like the Ampeg SVP Pro in the same class as a Vintech X73, for example, was out of line! :D

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